Artificial leathers have come to be widely used in the fields of clothes, general materials, sports, etc. because their light weights and easiness of handling have been accepted by consumers. In such application fields, high-quality artificial leathers which meet both the sensory feeling such as appearance and hand and the physical properties such as dimension stability are required. Artificial leathers excellent in appearance, hand, etc. are generally produced by a method including the conversion of microfine fiber-forming fibers into microfine fibers by removing one component therein. A generally known method of producing artificial leathers which includes the conversion of fibers into microfine fibers are roughly composed of a step (1) in which microfine fiber-forming fibers made of two kinds of polymers having different solubilities are made into staple fibers, a step (2) in which the staples fibers are formed into a web by using a carding machine, crosslapper, random webber, etc., a step (3) in which the fibers are entangled to one another by a needle-punching, etc. to form an entangled nonwoven fabric, a step (4) in which a solution or emulsion of an elastic polymer such as polyurethane is impregnated into the entangled nonwoven fabric and coagulated, and a step (5) in which the microfine fiber-forming fibers are converted into microfine fibers by removing one of their components. The step (4) and step (5) may be carried out in reverse order. By such production methods, soft artificial leathers composed of microfine fibers are obtained.
Unlike a method of using short fibers, a method of using long fibers does not need a series of large apparatuses such as a raw fiber feeder, an apparatus for opening fibers, a carding machine and a cross lay machine. In addition, a nonwoven fabric made of long fibers is advantageous to a nonwoven fabric made of short fibers because of its high strength.
Nonwoven fabrics of long microfine fibers have been mainly produced by a method in which microfine fiber-forming long fibers (also referred to as “composite long fiber”) composed of two or more kinds of incompatible polymers are made into a nonwoven fabric and then the microfine fiber-forming long fibers are converted into microfine fibers by splitting and dividing along the lengthwise direction through the interface between the polymers. However, it is very difficult to split or divide uniformly. Therefore, the obtained nonwoven fabric of long microfine fibers is mainly used in the production of grain-finished artificial leathers, and a nonwoven fabric of long microfine fibers which is applicable to the production of suede-finished artificial leathers is difficult to obtain.
Many artificial leathers having a softness like natural leathers have been proposed. One example thereof is produced by impregnating a polyurethane resin into an entangled nonwoven fabric of sea-island fibers, wet-coagulating the polyurethane resin, converting the sea-island fibers into bundles of microfine fibers with 0.2 D or less by removing the sea component by extraction with a solvent to obtain a substrate, applying a polyurethane solution on the surface of the substrate, wet-coagulating the polyurethane, and then gravure-coating a coloring composition to color the polyurethane resin (Patent Document 1). Although the proposed natural leather-like sheet has a flexibility which resembles that of natural leathers, a grain-finished artificial leather which combines the non-repulsive softness and firm hand each resembling those of natural sheep skins and fine buckling wrinkles have not yet been obtained.
Further proposed is an artificial leather having a flexible and dense feeling (firm texture) which is produced by impregnating a high-density nonwoven fabric with a resin in an amount smaller than usual (Patent Document 2). However, the proposed artificial leather has a surface with a poor softness and a low interlaminar strength. Therefore, it is insufficient as the material for support shoes which are used under severe conditions.
Still further proposed is a grain-finished artificial leather made of a nonwoven fabric of long fibers (Patent Document 3). In Patent Document 3, it is described that the strain markedly caused during the entangling treatment of long fibers can be relieved by intendedly cut the long fibers during the entangling treatment by needle punching, thereby exposing the cut ends of fibers to the surface of the nonwoven fabric in a density of 5 to 100/mm2. It is also described that 5 to 70 fiber bundles are present per 1 cm width on the cross section parallel to the thickness direction of the nonwoven fabric of long fibers, i.e., the number of fiber bundles which are oriented by needle punching in the thickness direction is 5 to 70 per 1 cm width of the cross section. It is further described that the total area of fiber bundles on an arbitrary cross section perpendicular to the thickness direction of the nonwoven fabric of long fibers is 5 to 70% of the cross-sectional area. Although cutting the long fibers to an extent achieving the intended properties, many long fibers are required to be cut to make the nonwoven fabric of long fibers into the proposed structure. Therefore, the advantages of using long fibers that the strength of nonwoven fabric is enhanced because of their continuity are significantly reduced, thereby failing to effectively use their advantages. To cut the fibers on the surface of nonwoven fabric evenly, the entangling treatment should be performed by repeating the needle punching many times under conditions severer than usual, thereby making it difficult to obtain the high-quality structure of long-fiber nonwoven fabric aimed in the present invention.
Patent Document 1: JP 63-5518B (pages 2-4)
Patent Document 2: JP 4-185777A (pages 2-3)
Patent Document 3: JP 2000-273769A (pages 3-5)